Early Life History of Blueback Herring and Alewife in the Lower Roanoke River, North Carolina

Abstract

Seasonal distribution, abundance, habitat use, hatch dates, growth, and mortality of larval blueback herring Alosa aestivalis and alewife A. pseudoharengus were studied in the Roanoke River watershed, downstream from the Roanoke Rapids Dam in North Carolina. Ichthyoplankton was sampled from man-made canals, a creek, an oxbow, a flooded bottomland hardwood forest, the main river channel, and along the river's edge. To identify larval clupeids, we developed a dichotomous key for clupeids found in the study area using morphometric, meristic, and pigmentation characters synthesized from the literature. Distribution, abundance, and back-calculated hatch dates of larvae and eggs were used to describe spawning period, spawning habitat, and nursery habitat. Blueback herring and alewife eggs and larvae were present from early April through late May. Based on the presence of eggs and early-stage larvae, both species spawned in backwater tributary systems, including flooded bottomland hardwood forests. Our results indicated that blueback herring and alewives used a variety of habitats during early larval stages, though interspecific differences in concentration among habitats were evident. Blueback herring used both lotic (moving water) and lentic (still water) habitats, and alewives were more abundant in lentic habitats. Larval growth varied between years but not between species. Blueback herring growth rates were 0.60 mm/d in 1996 and 0.42 mm/d in 1997 compared with growth rates of 0.65 mm/d in 1996 and 0.41 mm/d in 1997 for alewives. Interspecific and between-year differences in daily mortality of preflexion blueback herring and alewives (age, 4–8 d) were observed. Blueback herring daily percent mortality in 1996 and 1997 was 99%, while alewife rates decreased from 98% in 1996 to 91% in 1997. Fluctuations in river flow affected habitat use; moderate to high discharge rates increased use of spawning and nursery habitats, and low flows reduced use of spawning habitat.